1. Field of the Invention
This invention generally relates to electronic circuitry and, more particularly, to a system and method for an ultra low noise band gap reference voltage.
2. Description of the Related Art
Band gap—A well-known circuit used to provide a stable voltage reference;
PTAT—Proportional To Absolute Temperature; Band gap circuits generate a current that varies with the absolute value of the ambient temperature.
gm—transconductance of FETs and bipolar transistors;
C—Degrees Celsius;
K—Degrees Kelvin=C+273=absolute temperature.
As noted in Wikipedia, a bandgap voltage reference is a temperature independent voltage reference circuit widely used in integrated circuits, usually with an output voltage close to the theoretical 1.22 eV bandgap of silicon at 0 K. The voltage difference between two p-n junctions (e.g. diodes), operated at different current densities, is used to generate a proportional to absolute temperature (PTAT) current in a first resistor. This current is used to generate a voltage in a second resistor. This voltage in turn is added to the voltage of one of the junctions (or a third one, in some implementations). The voltage across a diode operated at constant current, or here with a PTAT current, is complementary to absolute temperature (CTAT—reduces with increasing temperature), with approx. −2 mV/K. If the ratio between the first and second resistor is chosen properly, the first order effects of the temperature dependency of the diode and the PTAT current cancel out. The resulting voltage is about 1.2-1.3 V, depending on the particular technology and circuit design, and is close to the theoretical 1.22 eV bandgap of silicon at 0 K. The remaining voltage change over the operating temperature of typical integrated circuits is on the order of a few millivolts. This temperature dependency has a typical parabolic behavior.
Because the output voltage is by definition fixed around 1.25 V for typical bandgap reference circuits, the minimum operating voltage is about 1.4 V, as in a CMOS circuit, at least one drain-source voltage of a FET (field effect transistor) has to be added. Therefore, recent work concentrates on finding alternative solutions, in which for example currents are summed instead of voltages, resulting in a lower theoretical limit for the operating voltage.
FIG. 1 is a schematic diagram of a low voltage band gap circuit (prior art). The circuit provides a reference current that tracks R2 and is relatively flat over temperature. The reference current, I3, can either be used directly as a bias current source or applied to a grounded resistor, and the voltage drop across the resistor can be used as a reference voltage. The minimum supply voltage is the voltage across diode D(1), plus VDSsat of the PFETs I1 or I2. This voltage can be as low as just under a volt.
The main disadvantage of this circuit is that it is noisy. The signal is the voltage drop across R2, which is equal to 26 mV*ln(N). The noise is the thermal noise across R2, which is the square root (sqrt) of (4 ktR) root mean squared (RMS), summed with the equivalent input noise of the amp RMS, and summed with the current noise of the current source FETs (sqrt(4*(⅔(kT*gm)))).
Since the signal is relatively small (26 mv*ln(N)), the noise of this circuit is considerable. The only way to reduce the noise in an integrated solution is to increase the current and area of the circuit. There are many variations of this circuit design.
FIG. 2 is a schematic drawing of a band gap circuit variation (prior art). The minimum supply voltage is the band gap voltage, 1.21V, plus the VDSsat of the current source FET 13. Thus, this circuit requires a supply voltage of over 1.3V. Though the current produced by this circuit has the same approximate noise level as the typical low voltage band gap circuit, the output voltage has a reduced noise level. The reason is that the output voltage noise is the current noise times the effective AC resistance of the load. With a typical low voltage bandgap circuit, the AC resistance of the load is simply the load resistance value. However, for a design like the circuit of FIG. 2, the AC resistance of the load is R3 plus 1/gm of the diode D.
For example, if I3 is 25 uA and the voltage is 810 mv, then R3 is 16K ohms and 1/gm of diode D=1540 ohms. So, the effective AC resistance is 17.54K ohms. However, if the same voltage level is provided by the low voltage band gap circuit of FIG. 1, the required resistance would be 1.21V/25 uA or 48.4 K ohms. Therefore, the band gap voltage reference of FIG. 2 is approximately 2.7 times lower in noise.
It would be advantageous if a band gap reference voltage circuit could be designed that operated at a low supply voltage, while supplying a low noise reference voltage.